Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) - USD ($)	Dec. 31, 2023	Aug. 21, 2023	Dec. 31, 2022	Aug. 20, 2021	May 26, 2021	Jun. 20, 2014	Nov. 22, 2006	May 18, 2004
Debt Instrument [Line Items]
Principal	$ 1,746,000,000		$ 1,726,000,000
Carrying Value	$ 1,694,000,000		1,675,000,000
Senior Notes | AGC | Senior Notes 6.125 Percent
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.125%	6.125%
Principal	$ 350,000,000	$ 350,000,000	0
Carrying Value	$ 345,000,000		0
Senior Notes | AGC | 3.15% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	3.15%
Principal	$ 500,000,000		500,000,000		$ 500,000,000
Carrying Value	$ 496,000,000		495,000,000
Senior Notes | AGC | 7% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	700.00%							700.00%
Principal	$ 200,000,000		200,000,000					$ 200,000,000
Carrying Value	$ 198,000,000		198,000,000
Senior Notes | AGC | 3.6% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	3.60%
Principal	$ 400,000,000		400,000,000	$ 400,000,000
Carrying Value	$ 395,000,000		395,000,000
Senior Notes | AGC | 5% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.00%					5.00%
Principal	$ 0		330,000,000			$ 500,000,000
Carrying Value	0		329,000,000
Senior Notes | AGMH | 3.15% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)					3.15%
Senior Notes | AGMH | 3.6% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)				3.60%
Enhanced Junior Subordinated Debentures | AGC
Debt Instrument [Line Items]
Principal	150,000,000		150,000,000
Carrying Value	150,000,000		150,000,000
Junior Subordinated Debentures | AGMH
Debt Instrument [Line Items]
Principal	146,000,000		146,000,000				$ 300,000,000
Carrying Value	$ 110,000,000		$ 108,000,000